The simple answer to my question (Davide256), something I hadn't noticed, is that the user has to start fidelizer manually each time, and reboot to get rid of it.
Now, being aware of that, there are a bunch of problems:
1) fidelizer kills foobar2000's volume control (Windows Sound Mixer is not an efficient way to conrol volume on the fly, to say the least);
2) How can one A-B or A-B-C (with JPlay or another player)?
a) it takes a minimum 45-60 seconds to turn off whatever else is on, if anything, and fidelizer in Pro mode, get their website opening out of the way, and then go back to the player and start the music (in Audiophile mode, fidelizer's install just ran on and on, except when JPlay had been part of startup, then stopped);
b) since fidelizer kills the player's volume control, time has to be spent adjusting (Window's) Sound Mixer, hoping to get close from memory;
c) since fidelizer does not show in processes or services, doing more than one comparison requires reboot and starting all over again (~2-3 minutes minimum?);
Under these conditions, I sense a difference with non-fidelizer vs. fidelizer, but it's hard to tell for sure or exactly what it is. Likewise, with JPlay (trial) vs. fidelizer, I sense that JPlay is more "atmospheric" than fidelizer, while fidelizer's tone or pitch is lower (something bothering me about JPlay is with its increased atmospherics, it also seems to raise the tone about "1/3" octave).
Now, being aware of that, there are a bunch of problems:
1) fidelizer kills foobar2000's volume control (Windows Sound Mixer is not an efficient way to conrol volume on the fly, to say the least);
2) How can one A-B or A-B-C (with JPlay or another player)?
a) it takes a minimum 45-60 seconds to turn off whatever else is on, if anything, and fidelizer in Pro mode, get their website opening out of the way, and then go back to the player and start the music (in Audiophile mode, fidelizer's install just ran on and on, except when JPlay had been part of startup, then stopped);
b) since fidelizer kills the player's volume control, time has to be spent adjusting (Window's) Sound Mixer, hoping to get close from memory;
c) since fidelizer does not show in processes or services, doing more than one comparison requires reboot and starting all over again (~2-3 minutes minimum?);
Under these conditions, I sense a difference with non-fidelizer vs. fidelizer, but it's hard to tell for sure or exactly what it is. Likewise, with JPlay (trial) vs. fidelizer, I sense that JPlay is more "atmospheric" than fidelizer, while fidelizer's tone or pitch is lower (something bothering me about JPlay is with its increased atmospherics, it also seems to raise the tone about "1/3" octave).